I'm trying to determine if such a tool exists. The problem I am trying to solve is as follows:

Imagine if - instead of designing $$ expensive asteroid probes like Dawn to visit specific asteroids (Vesta and Ceres) - you were sending numerous small probes as "mining prospectors" out into the solar system. Rather than having a large team working on one probe (with extensive DSN communications to and from that probe), you instead had a small team managing a fleet of largely autonomous probes that periodically send back imagery/data on the millions of asteroids that are out there.

Because these probes have to be relatively cheap and small, they don't have much in the way of thrusting capability, but they are built to last. Their aim is to "hop" from asteroid to asteroid, mapping each one for water/mineral resources as they go. Perhaps there is a significant degree of onboard processing which generates a model of each object, so that the data transfer back to Earth is minimised as much as possible. On those (presumably rare) occasions when two asteroids have a close approach, and the distance and velocity is within the capability of the probe to make the jump, it moves onto the next one. And the next one, and so on.

What I want is a tool where I can type in database IDs for two or more asteroids, and find out time and date of the close approach as well as the approach velocity, exactly as is calculated for NEOs approaching Earth. It doesn't have to be particularly accurate (simple orbital elements should suffice - I'm not wanting to make something too computationally expensive) - I just want to get a feeling for how frequent or rare these close approaches actually are. Sure, space is huge, but there are also lots of asteroids (many of which are grouped into families with similar orbits), and I'm surprised I haven't been able to find such a tool.

It indicates the minimum dV for a specified mission duration (or for a minimum mission duration) from Earth. While this is not particularly useful for what you're looking for, a script could be developed that looked at similar accessibility dates and a delta-delta-v (i.e. the difference in dV for a given mission duration as output by the tool) under a certain threshold. This should give you NEAs with low-impulse relative accessibility requirements, or a rough approximation to low relative velocity.

Of course, this doesn't take into account their relative *distance*, but it could be a start from where to weed some of them out. A second script could use the ephemeris database to gather their relative distance and discard those over a certain distance threshold, avoiding to have to finely determine in time all asteroids' orbits.